In the manufacture of printed circuits by a screen printing technique, a circuit pattern is formed in a screen by filling selected mesh openings of the screen with a fluid material which solidifies. The unfilled mesh openings thereby form the desired circuit pattern.
In one method of using the screen printing technique, a substrate, having copper cladding on at least one surface, is placed on a platen. The pattern-containing screen is placed over the other substrate surface which contains copper cladding. Ink is flooded over the entire exposed surface of the screen but is not forced through the circuit-pattern openings thereof. A squeegee is then moved over the ink-flooded side of the screen to force ink through the circuit-pattern openings and onto the copper cladding of the substrate to define thereon the inked circuit pattern. The squeegee also wipes the excess ink into a pool or reservoir on one side of the top of the screen for use in the next flooding operation. The screen is then raised to reveal the substrate with the inked circuit pattern on the copper cladding. The pattern-inked substrate is then processed in a conventional manner to form a printed circuit thereon.
A screen printer which performs in the foregoing manner is commercially available from The Dia-Print Company, Inc. of Phoenix, Ariz. and is identified as an Itron Printer, Model TNTW.
Specifically, the Itron Printer includes an open-top housing which supports the circuit-pattern screen in the bottom thereof. A platen is located on a substrate-support table and is positioned below the housing. A pair of locating pins are secured to the underside of the housing and are seatable within locating apertures in the table to facilitate accurate positioning of the screen relative to the platen. A cross arm extends between opposite sides of, and within, the housing. The arm is secured at the ends thereof to a chain-drive mechanism which facilitates motor-driven movement of the arm between the front and rear of the housing in either direction. An air cylinder is mounted on the arm to facilitate partial rotational movement of the arm about its axis. A flooding bar and a squeegee are supported along the length of the arm and extend between the sides of the housing over the screen.
In use, a supply of ink, in a semi-viscous state, is deposited onto a portion of the screen near the front of the housing to form the pool or reservoir. The air cylinder is controlled to pivot the arm so that the flooding bar is close to the screen and in position adjacent to the deposited ink. With the arm in this position, the squeegee is positioned sufficiently above the screen and ink so that it will not engage the ink. The arm is then moved to the rear of the housing whereby the flooding bar spreads or floods the ink over the screen but is sufficiently above the screen to avoid forcing ink through the circuit-pattern openings of the screen.
A copper clad substrate is positioned on the platen and the housing is lowered whereby the locating pins seat in the table openings to accurately locate the platen with the screen. The air cylinder partially rotates the arm to move the flooding bar substantially above the screen and the flooded ink and to move the squeegee into the ink-wiping position. The arm is then moved toward the front of the housing whereby ink is wiped or forced through the circuit-pattern openings and onto the copper clad substrate. The squeegee wipes the excess ink into the pool or reservoir adjacent the front of the housing in preparation for the next flooding operation. The housing is then raised and the substrate containing the inked pattern can be removed. The substrate is then processed in a conventional manner to form printed circuits on the substrate.
In a method of manufacturing flexible printed circuits using the screen printing technique, circuit patterns are screen-linked directly onto a surface of copper cladding of successive sections of a flexible web with land areas of the inked patterns desirably located about positions whereat holes have been punched in the cladding and through the web. Thereafter, those portions of the copper cladding which are not covered by the ink are removed in an etching process. The ink is then removed from the remaining copper cladding and additional copper is deposited onto the now-exposed cladding, onto the walls of the now punched holes and, if desired, onto similarly formed circuit patterns of copper cladding on the opposite side of the flexible web.
To facilitate the screen printing process, a series of through slots are placed in spaced relation in each of successive sections of the flexible web along one margin thereof in a precise relationship to the positions whereat holes are simultaneously punched through the web. In addition, slot locators are formed in the screen in precise relationship to circuit patterns in the screen and are designed to coincide with the through slots of the flexible web when the screen and successive web sections are subsequently aligned.
A light source is aligned with the slot locators in the screen and, but for the presence of the flexible web, would normally direct light rays onto light sensitive devices, such as photo-diodes, which are mounted in a sensor bar movably positioned between the screen and the web but also secured to the screen for movement therewith. The light sensitive devices are connected to a control system to assist in the control of a pair of web feed drums, at entrance and exit locations to the screen, which cooperate to accurately position the flexible web under the sensor bar and to simultaneously move the screen therewith. Thereafter the sensor bar is retracted independently of the screen and a screen positioning mechanism is controlled to accurately position the circuit pattern of the screen relative to the flexible web.
In operation, successive sections of the flexible web are indexed adjacent to the sensor bar and the screen and each section is accurately aligned with the photo-diodes for the screen printing operation. As each section of the flexible web is indexed into a position adjacent to the screen, a first of the slots in the web passes one of the light sensitive devices which is activated to facilitate the slowing of the feed of the web. Thereafter, the same slot in the flexible web is positioned for alignment with another of the light sensitive devices which responds and develops a voltage to control the synchronous operation of the feed drums and accurately align the slot with the light sensitive device so that the section of the flexible web is aligned accurately in the longitudinal direction with the ultimately positioned circuit pattern on the screen.
As the first slot is positioned for alignment with the corresponding light sensitive device, two other slots in the flexible web are also positioned to be aligned with corresponding light sensitive devices. When these slots and corresponding light sensitive devices are generally aligned, the devices respond to develop voltages which provide for the movement of a support jointly for the light sensitive devices and the screen to precisely align the two other slots with the corresponding light sensitive devices and thereby provide for the precise lateral alignment of the circuit pattern with the section of the flexible web.
After the two-direction alignment is accomplished, the sensor bar is retracted independently of the screen. A table which is positioned adjacent to the web on the side opposite from the screen, vacuum grips the section of the flexible web and moves the web section to a spaced position beneath the screen. Thereafter, ink is deposited onto the screen and subsequently wiped through the pattern and onto the copper cladding of the web.
A reverse procedure is initiated to release the gripped web section and the next successive section of the web is then moved to align the next successive section with the circuit pattern for the next ink printing operation.
A screen printing system similar to the type described above has been described in the "Description of the Prior Art" section of U.S. Pat. No. 4,109,158 which, by reference thereto, is incorporated herein.
The above-described apparatus for performing the screen printing operation in the manufacture of flexible printed circuits involves the manipulation of the housing which supports the screen and the facilitates for flooding and wiping the ink over and through the pattern in the screen. In the controlled manipulation of the housing, surfaces of the housing must slide over fixed support surfaces to facilitate the accurate aligning of the slots of the web and the corresponding slot locators of the screen to precisely locate the circuit pattern over the web prior to the screen printing operation. During the positioning period, static friction between the housing surfaces and the support surfaces must be overcome before positioning movement is attained. This results in a time lag where the movement of the housing is not immediately responsive. Further, once the static friction has been overcome, the moving force applied to the housing may be excessive for the critical positioning of the screen and results in "overshooting" of the desired position. Thus, damping compensation must be provided to reduce the effects of such "overshooting."